Device for controlling the means delivering sheets to a machine

ABSTRACT

The device comprises a lifting mechanism ( 1 ), a sheet-inserting means, means for detecting the front top level of the stack ( 32 ) connected to a computer acting on an electric motor ( 11 ) of the stack-lifting mechanism ( 1 ), and means for raising the stack ( 4 ). The detector ( 32 ) of the front level of the stack ( 4 ) comprises means for measuring the extent of variations in the level of the stack. The computer is also connected to a source of information ( 17 ) relating to the supply frequency in real time and to the nominal thickness of the sheets. The computer output is connected to a frequency-varying means for controlling the electric motor ( 11 ), the computer being designed so that the value of the signal appearing at its output is characteristic of the difference between the measured level of the stack ( 4 ) and the calculated level.

[0001] The invention relates to a device for controlling the meansdelivering sheets from a stack to a machine for processing them.

[0002] The operation of successively taking the top sheet from a stackof sheets to form a layer of sheets for insertion into a printing orcutting machine is well-known and numerous existing devices serve thispurpose. In the case of a sheet-by-sheet supply of solid fibre board orcorrugated board, flatness is a frequent problem, since the top surfaceof the stack may vary considerably in level. This is because thethickness of a cardboard sheet may vary in dependence on a number ofexternal factors such as the ambient humidity or the storage conditions,the result being a difference in height between the front and rear ofthe stack and also between the centre and edges thereof.

[0003] It has already been proposed to obviate this disadvantage bymeans of a device described in CH 651 807, comprising a mechanism forlifting the stack, a sheet-inserting means comprising a grippingelement, means for detecting the top level of the stack comprising adetector of the front level of the stack connected to a computer actingon an electric motor of the stack-lifting mechanism and means forraising the stack in dependence on the said top level. When the frontlevel detector cannot see the stack of sheets, a command pulse forraising the stack is sent to the driving motor, which sends a pulsedriving the stack-lifting mechanism. Depending on the thickness of thestack of sheets, the duration of the pulse is varied to obtain avariable rise step, so as to bring the top surface of the stack to thesame level after the departure of each sheet from the stack.

[0004] A device of this kind considerably improves the accuracy of thetop level of the stack, more particularly of the front vertical surface,but all the same, the disadvantage of raising the stack of sheets bypulses is that the least offset between successive pulses may result ina variation in the step or pitch of the resulting layer of sheets. Ifthe step tolerance exceeds a certain amount, the machine processing thelayer of sheets stops and has to be restarted, resulting in substantialloss of production.

[0005] The object of the invention is at least partly to obviate thisdisadvantage.

[0006] To this end, the invention relates to a device for controllingthe means delivering sheets from a stack to a machine for processingthem as defined by claim 1.

[0007] The accompanying drawings, diagrammatically and by way ofexample, illustrate an embodiment of the control device according to theinvention.

[0008]FIG. 1 is a sectional view of a sheet supply station and

[0009]FIG. 2 is a block diagram of the control means of the mechanismfor raising the stack for supplying sheets.

[0010]FIG. 1 shows a supply station of the kind described in CH 651 807,to which reference should be made for further details. It comprises astack lift comprising a lifting grid 1 suspended on chains 2. One end ofeach chain 2 is attached to the lifting grid 1 via at least two,preferably four, lugs 3 placed on either side of the sheet-supplyingstack 4. The other end of each chain 2 is connected to an attachmentmember through which a screw extends. Each chain 2 passes around a chainwheel 7 mounted on shafts 8. A device vertically drives the lifting grid1 via the chains 2. The device can comprise the attachment member which,of course, is guided by slides (not shown) in its motion along thescrew. The screw is held by two bearings (not shown) and driven by ageared motor 11 equipped with a coder 41. Of course, use can be made ofany geared motor device capable of rotating the shaft 8 secured to thechain wheels 7.

[0011] The insertion station also comprises a suction group 12 fixed toa chassis 13 suspended from the end of at least one or in the presentcase at least two chains 14 via lugs 15. The other ends of the chains 14are fixed to the output shaft of a linear motor 18, where the chains 14pass around chain wheels 22 mounted on a transverse shaft 23. One end ofthe chassis 13 rests in slidable manner on a cross-member 24 whereas theother end is provided with a guide device mounted in holders 27 fixed toeach lateral upright 28 of the frames 47, 48 of the supply station. Thisarrangement ensures that the position of the suction group 12 does notchange relative to the rear edge of the stack of sheets 4 when it israised or lowered by the action of the motor 18. The suction group 12comprises a number of suckers 29 (to simplify the drawings, only one hasbeen shown) and a first detecting means 30 which will be described infurther detail hereinafter. The suction group 12 is described only inorder to show how sheets are supplied to the machine; this known suctiongroup 12 is not part of the invention and will not be described indetail here.

[0012] The first detecting means 30, which comprises a cabriole foot 19and a scanning cell 20, is placed near the rear top part of the stack ofsheets 4 along a theoretical axis corresponding to the theoreticalcentral axis of the stack of sheets 4. A second detecting means 32 isplaced opposite the front top part of the stack of sheets 4 along thecentral axis of the stack. The second detecting means 32 preferablycomprises a fixed linear camera disposed in the vertical direction ofthe stack of sheets 4, so as to supply an analog signal depending on themeasured level.

[0013] A retractable front abutment 32 is mounted so as to be pivotablearound an axis 34 near the top level of the front surface of the stackof sheets 4. The front abutment 33 is pivoted by a lever 35 controlledby a cam 36 actuated by the device driving the insertion stationsupplied by sheets from the pile 4. The abutment 33 has a window (notshown) centred along the central axis of the stack of sheets 4, givingthe detector 32 a constant view of the top level of the stack 4.

[0014] The first detecting means 30 is electrically connected to one ofthe circuits of a computer 37 by a cable 38, whereas the seconddetecting means 32 is connected to a second circuit of the said computer37 by a cable 39. When the computer 37 receives a signal from the firstdetecting means 30, it transmits positive or negative information fortransmission by a cable 40 to the motor 18 controlling the verticalmotion of the suction group 12.

[0015] As illustrated by the block diagram in FIG. 2, information otherthan that supplied by the second detector 32 is supplied to the computercircuit 37 in order to control the speed of the geared motor 11. Theinformation relates to the speed of the machine, given by the valuestransmitted by a pulse generator 17 indicating the frequency at whichthe suction group 12 raises the sheets from the stack 4. The thicknessof the cardboard sheets constitutes the other information introducedinto the computer 37. These two items constitute the data fordetermining the theoretical speed at which the geared motor 11 shouldraise the stack of sheets 4. This speed, calculated on the basis of thesaid data, is the set speed of the geared motor 11. The analog signalsupplied to the calculator by the second detector 32 enables it in realtime to evaluate the difference between the top level of the stackmeasured by the second detector 32 and the reference top level of thesaid stack. The comparison between these two levels is used to form aspeed signal at the computer output, which is compared with the speed inreal time of the geared motor 11 supplied by a coder 41 associated withthe motor. The result of the comparison between these two speed signalsis used to increase or decrease the supply frequency of the motor, via afrequency varying means 42 connected to the motor 11. The motor 11 thuscontinuously raises the stack of sheets 4 at a speed constantly adaptedto the real level of the stack compared with the reference level.

[0016] The insertion station may also comprise an auxiliary supplydevice 50, commonly called non-stop (NS), for changing a stack withoutinterrupting the flow of sheets. The auxiliary supply device 50comprises a frame 51 for positioning bars 52 for holding the stack whenthe grid 1 lifting the main stack has reached a level requiringreloading with a new stack of sheets. Accordingly the bars 52 areinserted under the main stack so as to form an auxiliary grid whichholds the main stack and enables the grid 1 to be lowered to a bottomposition for loading the new stack.

[0017] The non-stop device generally comprises a drive having amechanism similar to that for raising the grid 1. Inter alia itcomprises a geared motor 53 for driving chain wheels 54 around whichchains 55 pass and vertically move the frame 51. The geared motor 53 isalso connected to the computer 37. When the sheets are supplied by thesuction group 12 from the auxiliary supply device 50, the liftregulating means used for the grid 1 are used for the auxiliary supplydevice 50.

1. A device for controlling the means delivering sheets from a stack (4)to a machine for processing them, comprising at least one mechanism (1)for lifting the stack, a sheet-inserting means comprising gripping means(29), means for detecting the upper level of the stack comprising adetector of the front level (32) of the stack (4) connected to an inputof a computer (37) acting on at least one electric motor (11) of thestack-lifting mechanism (1), and means for raising the stack (4) independence on the upper level, characterised in that the detector of thefront level (32) of the stack (4) comprises means for measuring theextent of variations in the level of the stack and in that an input ofthe computer (37) is also connected to a source of information (17)relating to the supply frequency in real time and to the nominalthickness of the sheets, and the output of the computer (37) isconnected to at least one frequency varying means (42) for controllingthe electric motor or motors (11), the computer (37) being designed sothat the value of the signal appearing at its output characterises thedifference between the measured level of the stack (4) and the levelcalculated on the basis of the said nominal thickness and the saidsupply frequency and generates a variation in the frequency of the oneor more varying means (42) tending to modify the rate of advance of themotor (11) in order to make the measured level coincide with a setlevel.
 2. A device according to claim 1, characterised in that thedetector (32) of the front level of the stack (4) is a linear cameradisposed in the vertical direction of the stack.
 3. A device accordingto claim 1, characterised in that the lifting mechanism (1) isassociated with an auxiliary supply means (50) controlled by a motor(53) connected to the computer (37) such that its rate of advance isdetermined by the same information as used for controlling the one ormore electric motors (11) of the lifting mechanism (1).